The notion that the human immune system might be able to detect and eradicate cancer cells has been accepted as well as rejected multiple times in the last century. As early as 1909, Paul Ehrlich assumed that the immune system might be involved in the detection of malignant cells, an idea that was later expanded on by Burnett's and Thomas' hypothesis of “immunosurveillance”, stating that the immune system not only observes but is also capable to eliminate these cells. These concepts then became widely unpopular during the 60s and 70s of the last century due to several studies that were unable to show a significant difference in the incidence of tumors between immunocompetent and immunodeficient mice.
Later on, this was again superseded by results now showing an influence of the immune system on the development and the progression of malignant diseases with T cell-mediated immunity playing an important role. Accordingly, it was found that tumor infiltration by T cells and their respective proliferation rate constitute a positive prognostic factor in renal cell [Nakano, O., et al., Cancer Res, 2001, 61(13):5132-6], ovarial, uterine and colon carcinoma, as well as multiple hematologic malignancies, such as Non-Hodgkin lymphoma.
In parallel, knowledge in another domain of tumor immunology, the identification of specific tumor-associated antigens, grew just as rapidly. For a long time, only few tumor antigens were known and thus available for the development of immunotherapeutic agents. This was remedied by an increased scientific effort targeted towards the development of new methods, such as SEREX and T cell epitope mapping, focusing exclusively on the identification of potential immunologic target structures during the last 15 years. The multitude of novel tumor antigens described by these techniques was classified according to their origin, function or expression pattern in comparison to healthy tissues, such as overexpressed antigens or cancer-testis (CT) antigens, with the latter representing an especially promising group of antigens possibly suited for the development of T cell based therapeutic strategies [Simpson, A. J., et al., Nat Rev Cancer, 2005, 5(8):615-25].
The identification of tumor antigens as target structures for vaccines, antibody therapy or adoptive immunotherapy remains a major goal in the field of tumor immunology. Essential for their use in these settings are a homogenous expression in the tumor, easy accessibility, and highly restricted expression patterns in healthy tissues. CT antigens, a gene family currently containing more than 130 members, are commonly characterized by such specific expression patterns, typically limited to the testis and undetectable in other normal human tissues. A high proportion of CT antigens is located on the X chromosome and these antigens seem to show a particularly tumor-specific expression.
Another distinct property of this group of antigens is their natural immunogenicity, a feature that led to the identification of a large number of members of this gene family via autologous typing. Importantly, CT antigen expression has been described in numerous cancers with widely differing origins and was repeatedly found to be associated with disease progression and loss of differentiation in cancer cells. It has been hypothesized that CT genes might contribute to the appearance of therapy-resistance and, as a consequence, persistence of residual disease in the case of human cancers. Supporting this idea, recent studies have shown that expression of MAGE and GAGE genes in cancer cell lines derived from solid tumors induces a chemotherapy-resistant phenotype in vitro and tumor expression of MAGE-A1 seemed to correlate with clinical responses to taxan-based chemotherapy in gastric cancer patients. Furthermore, it has been hypothesized that expression of CT genes, which are often heterogeneously expressed in tumor tissues by only a certain proportion of cells within the tumor mass, might represent a hallmark of cancer stem cells [Lee, S. Y., et al., Proc Natl Acad Sci USA, 2003, 100(5):2651-6]. These and other results indicate an important biological role of CT antigens for the malignant phenotype. Tumor-specific proteins with a central function in the promotion of the malignancy might represent particularly attractive targets for immunotherapy since (1) the tumor cannot “afford” to down-regulate them under the pressure of immune selection and (2) targeting the cells expressing the given protein might specifically eradicate those cells that guarantee tumor survival and growth.
Unfortunately, despite the remarkable tissue specificity and the distinctive immunogenicity of CT antigens, the development of targeted therapies employing this interesting group of genes has been hampered by the fact that essentially all members of this protein family are limited to the cytosol. Based on this fact, CT antigens have been considered “invisible” to antibody-mediated immune effector mechanisms. Ideally, a CT antigen useful for future immunotherapeutic approaches involving monoclonal antibodies would be naturally located on the malignant cell's surface.
In 2003, several cancer-testis antigens, among them the antigen NY-SAR-35 or, named according to its gene locus FMR1NB, were discovered by Lee et al. of the Ludwig Institute for Cancer Research in a SEREX analysis of sarcoma patients [Lee, S. Y., et al., Proc Natl Acad Sci USA, 2003, 100(5):2651-6; WO 2004/031354]. Following its initial description, this antigen has not been subject to any further investigation either in vitro or in vivo. WO 2004/031354 teaches that NY-SAR-35/FMR1NB represents a newly defined CT antigen expressed exclusively in normal testis, melanoma, sarcoma, lung cancer and breast cancer.
Hematological malignancies are a separate group of cancers, which are derived from cells of a different developmental origin than the cancers previously mentioned. In particular, they originate from blood cells and bone marrow cells as well as immune cells within lymph nodes. Acute myeloid leukemia, AML, is one example of a hematologic malignancy, other diseases from this group comprise chronic myeloid leukemia, acute lymphatic leukemia, chronic lymphatic leukemia, Hodgkin's disease and Non-Hodgkin lymphoma as well as multiple myeloma and myelodysplastic syndrome (MDS). Myeloproliferative diseases are related diseases. While there are treatment options for some of these diseases, further therapeutic approaches are urgently needed.
Acute myeloid leukemia (AML) describes a group of clonal, hemato-logic malignancies that are characterized by the malignant transformation of distinct cell stages from the maturation of hematopoietic progenitor cells. The accumulation of acquired genetic aberrations on one hand leads to a developmental arrest of these pluripotent cells, on the other hand, it promotes uncontrolled proliferation, such as the proliferation caused by mutations in the genes coding for N-Ras and FLT3. AML is the second most common leukemia in adults comprising approximately 90% of acute leukemias with an incidence of 3.6/100.000. Generally, AML can be observed in patients of all ages, but it is characteristically associated with increased age, as indicated by a median age at the time of diagnosis of 65 years. In these patients, mean overall survival is approximately 11 months with a 5-year survival rate of only 20%. Several risk factors contributing to the development of secondary AML have been identified, including chemo- and radiotherapy, as well as a pre-existing myelodysplastic syndrome.
Initial clinical characteristics are usually unspecific symptoms associated with the replacement of the physiological hematopoiesis, such as anemia, impaired coagulation, and frequent infections. At the time of diagnosis of AML, leukopenia might be observed in the peripheral blood of patients, and diagnosis of AML is therefore considered proven only upon the presence of 20-30% of myeloblasts in the bone marrow. Untreated AML will inevitably lead to death within few weeks. The mode of therapy is determined by the FAB (French-American-British) subtype, as well as several prognostic factors such as cytogenetics and the presence of certain molecular markers, but most commonly consists of induction therapy with Daunorubicin or Idarubicin and Cytarabine. Age remains the most important predictor of therapeutic success. Patients younger than 60 years show a 25% higher probability to achieve remission and show a significantly reduced rate of relapses and therapy-related mortality. However, without further therapy, almost all patients will eventually experience relapse following initial remission. Therefore, consolidation treatment (post-remission therapy) is recommended for all patients to extend the duration of the disease-free survival. Consolidation therapy is less uniform than induction therapy and consists of numerous protocols involving different chemotherapeutic options or different modes of stem cell transplantation. However, even by using such an extensive chemotherapeutic regimen, long-term disease-free survival is only achieved in 20-30% of patients.
Another major challenge in the treatment of AML is still posed by the predominantly old and frequently co-morbid patient collective. Only 30% of patients older than 60 years receive conventional induction therapy due to therapy-related mortality in up to 50% of these patients. Although highly selective inclusion criteria were formulated for this group, even patients who receive conventional treatment show vastly reduced remission rates. In addition, subsequent performance of allogeneic stem cell transplantation remains problematic, despite improved non-myeloablative conditioning regimens. Causes for this phenomenon include the lower probability of HLA-matched family donors in older patients and an increased risk for Graft-versus-Host disease.
In the U.S., Gemtuzumab-Ozogamicin, an antibody binding to the lektin CD33 linked to a toxin (WO 2004/043344), has been admitted in consolidation therapy for patients over 60 years of age, where conventional chemotherapy is not indicated. However, approval in Europe was refused due to allegedly insufficient data and an unclear risk-benefit ratio.
To address the severe problems in the current treatment of AML, the development of alternative and more targeted approaches that can be safely applied in such settings is essential. Furthermore, it would be beneficial to develop a means of diagnosis that only requires a blood sample from the patients.